High-frequency dielectric heating



Oct. 4, 1949. R. M. BAKER HIGH-FREQUENCY DIELECTRIC HEATING Filed Sept.28, 1944 2 Sheets-Sheet 1 n e K a 5 M U 0 PM BY 6 zzwu ull ATTORNEY WITNESSES:

Oct. 4, 1949, R. M. BAKER HIGH-FREQUENCY DIELECTRIC HEATING 2Sheets-Sheet 2 Filed Sept. 28, 1944 lZO MW M Wm W x, 5

WITNESSES:

Patented Oct. 4, 1949 UNITED STAT ES PATENT OFFIC E HIGH-FREQUENCYDIELECTRIC HEATING Robert Baker, 'PittsburghfiPa assignor toWestinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof liBennsylvania Application September 28, 19.44, Serial No. 556,140

.10 Claims.

length, "perhaps as much as twenty feet or more.

At the operating frequencies, standing-wave voltage patterns are likelyto exist along the heating-electrodes and also on the lines conveying ortransmitting power to them. If there is enough difference between themaximum and minimum values of the standing-wave patterns on theheating-electrodes, serious variations :oc'curin the manner in whichdifferent parts of the material are heat treated. Such non-uniformity ofheat treatment maybe further aggravated if the power delivered perincrement of electrode length or surface is large so "as to cause anattenuation of the standing-wave pattern 'on the heating-electrodes fromthe point at which the power is delivered. For production on acommercial scale, the troublesome problem arises of providing atube-oscillator generator having a tunable resonant plate circuit ortank circuit which can supply the power required.

An object of my invention is to improve the manner in which heat isdielectrica-lly introduced into a material which is dielectricallyheated between heating-electrodes having a part of a standing-wavepattern therein.

'A further general object of my invention is to provide a dielectricheating system of a type described in which the standing wave-pattern onthe heating-electrodes can be adjusted so as to provide a desired partof a wave length therealong.

A particular object of my invention is to provide a dielectric heatingsystem having heatingelectrodes of an electrical length whichencompasses an appreciable part of a standing wave, but which will heatthe dielectric material with satisfactory uniformity although thestanding wave has widely different maximum and minimum values on theheating-electrodes.

Another object of my invention is to provide a high-frequency dielectricheating system by means of which considerable energy can be quickly andsubstantially uniformly put into a large piece of material.

important object of my invention is to provide a voltage distributionover the heating-e1ectrodes which is intermittently changed during aheat-treatment cycle of the material being heat treated. The-voltagedistribution is changed in a manner to cause the square of the differentvoltagesacrcss substantially-opposite parts of the electrodes to besubstantially the same, or within close variations, when averaged overthe -heat treatment cycle for the material.

A feature of my invention resides in providing a dielectric heatingsystem having one or more tubeoscillator power supplies in which thecustomary tarikcircuit is replaced by a power transmission linecomprising a frequency-determining circuit.

Other features, objects, methods, innovations and combinations of myinvention will be discernible from the following description thereof,which is "to be taken in conjunction with the accomp'anyingdrawingsillustrating forms of my invention at present preferred. In thedrawings,

Figure l is-adiagram-matic'view of an embodimentof m-yinven-tion withparts in elevation;

Fig. 2 is a top plan View of a portion thereof illustrating plate-likeconductors used in the power transmitting lines;

Fig. *3 is a curve illustrating a standing-wave voltage patternestablished on the power lines and heating-electrodes under differentconditions of operation of the embodiment of my invention which is shownin Fig. 1, the ordinates representing the voltage-magnitudes and theabscissae representing electrical lengths in degrees;

Fig. 4 a--diagra-mmatic'view of a second embodiment of my invention; and

Fig.6 is a curveillustrating the relative heating along theheating-electrodes of the embodiment of Fig. 4, under differentconditions of operation.

In'carry-ing out my invention, I provide a dielectr'ic heating equipmentcomprising heatingelectrodes to the opposite ends of which sets ofsimilar conductors are electrically associated or connected. The otherfar ends of the conductors are alternately energized with high-frequencypower or short-circuited, the far ends of one set of comzluctors beingshort-circuited when the other is delivering power to theheating-electrodes, so that each set of conductors functions as a powertransmission line, and is so designated herein. In the embodiment shownin Fig. 1, the heating eleetrodes and transmission lines are coextendingand a separate tube-oscillator generator is provided for each of thetransmission lines; while in that shOWn in Fig. 4 the transmissionline'conductors extend upwardly so as to save floor space-and to permit:a single tube-oscillator to be selectively connected to each of thetransmission lines.

Referringto Fig. 1, a dielectric heating means is indicated in itsentirety by the reference numera12. It comprises a -plurality ofsubstantially equally spaced rectangular heating-electrodes includingouter grounded plates or platens 4 and 6, and an intermediate plate 8,all of substantially the same dimensions. The outer heating-electrodesare pressed toward each other and toward the intermediate electrode, asindicated by the arrows shown, for applying pressure to dielectricmaterial l which is to be treated under heat and pressure, In theparticular instance, this material comprises a plurality of alternatinglayers of veneer l2 and glue M for making plywood.

The heating-electrodes 4, 6 and 8, have opposite end sides, It and [8,to which a transmission line, and 22, respectively, is conductivelyconnected in any suitable manner. Each transmission line comprises apair of grounded outer conductors 24 and 2B and an intermediateinsulated conductor 28, in parallel relation. The conductors are ofsheet copper and substantially fiat, having a width in the preferredembodiment equal to about the width of the heating-electrodes. Althoughnot absolutely essential, it is preferable to keep the conductorsuniformly spaced a distance which is related directly to the spacing ofthe heating-electrodes, and inversely to the square root of thedielectric constant of the material between the heating-electrodes.

Power is established on the transmission lines 25 and 22 bytube-oscillator generators and 32, respectively, located in groundedmetallic cages 34 and 33, respectively, at the far ends of thetransmission \lines which are away from the heating-electrodes. Eachcage is provided with an open window for the associated insulatedconductor 23 of the associated transmission line. The other groundedconductors are grounded to the corresponding cage. Each of thetube-oscillator generators is shown schematically as comprising anoscillator tube 38 having an anode or plate-electrode 48 electricallyconnected or associated, through a coupling means comprising a blockingcapacitor 42, to the insulated conductor 23.

With a dielectric material l0 between the heating electrodes, bydielectric meaning one having a loss factor in the order of .1 or less,or even more under some circumstances, the impedance characteristicsbetween the conductor 28 and the grounded conductors 24 and 26 usuallywill be the primary factor in determining the frequency at which thetube 38 oscillates. The tube 38 includes a control electrode or grid 44to which a tunable grid circuit is connected so that a tuned platecircuit, tuned grid circuit form of oscillator is provided.

The grid circuit includes an inductor 46 connected in parallel with aVariable capacitor 48, the parallel circuit having one side seriesconnected to one end of a biasing resistor 50, the other end of which isconnected to a 10W resistance branch-circuit 52 which includes a movablecontact 54 of a relay means 56. When the movable contact 54 is in itsback or closed position, it completes the low resistance branch-circuit52 and permits the tube 38 to oscillate and deliver a high power derivedfrom the D. 0. plate energizing circuit 58 which includes a highfrequency choke 6i) and a plate battery 62. When the contact 54 is inits front or open postion, the circuit 52 is interrupted and a negativebiasing branchcircuit 64 is effective on the grid circuit for applying asufiiciently high negative bias to the grid 44 of the tube 38 forpreventing oscillations. A protective resistor 66 is included in thebranchcircuit 64 for preventing excessive current when thebranch-circuit 52 is closed at the contact 54. Accordingly, it isobvious that when a contact 54 is in back closed position, theassociated tubeoscillator generator 38 or 32 is conditioned forgenerating high frequency power for application to the transmissionline-section 20 or 22, and when it is in the front position oscillationsare prevented because of the high negative bias applied to the grid.

In accordance with my invention, it is desired to short-circuit each endof the transmission lines 20 and 22 at suitable times. Forshort-circuiting purposes, one of the grounded conductors 24 of eachtransmission line is provided with a hinged conductor section 10 havinga contact 12 adapted to engage a contact 14 at the end of the insulatedconductor 28 when the relay means 56 associated therewith is energizedfor placing its contact 54 in front or open position.

In order to control the sequence of operations of the apparatus, Iprovide a timer 76 adapted to rotate once every twenty or thirtyseconds, although the time period is variable over a wide range. Thetimer 16 comprises a grounded conducting segment 13 of somewhat morethan a half circle and an insulating segment 80 for the balance of thecircle. Brushes 82 and 84 are provided on diametrically opposite sidesof the timer. During rotation of the timer, a circuit is alternatelycompleted to the relay means 56 of the tube-oscallator generator 38 andto the relay means 56 of the tube-oscillator generator 32.

The operation is as follows: Assuming the apparatus in the positionshown, a circuit is completed to the operating coil 86 of the relaymeans 56 of the tube-oscillator generator 32. This circuit starts fromone end of a battery 88, continues through a conductor 90 to the coil86, through a conductor 92, the brush 82 and conducting segment 18, tothe other grounded end of the battery 88. Inasmuch as the brush 84 is onthe insulating segment 80, the circuit from the battery 80 through theconductor 94, the operating coil 96 of the relay means 56 of thetube-oscillator generator 30, the conductor 98,

and the brush 84, is interrupted so that this relay means isdeenergized.

With the operating coil 86 energized, the grounded hinged conductorsection I8 is in raised position short-circuiting the transmission line22, and the associated contact 54 is in open position permitting thelarge negative bias to be applied to the grid of its associated tube 38so that the tube-oscillator generator 32 is not in oscillatingpower-delivering condition. On the other hand, the tube-oscillatorgenerator 30 is in oscillating condition because its operating coil 96is deenergized so that its associated contact 54 is in closed positionpermitting its associated tube 38 to oscillate, and the hinged conductorsection 10 for the transmission line 20 is in down position, withcontacts 12 and 14 separated, so that the high frequency power is sentalong the transmission line 20.

When the timer moves in the direction of the arrow, the brush 84 firstengages the conducting segment 18 before the brush 82 leaves thissegment. Accordingly, the operating coil of the tube-oscillatorgenerator 30 is energized, and its contact 54 is first moved to openposition and then its grounded movable conductor-section 10 is connectedto the insulated conductor 28 of the transmission line 20. Bothtube-oscillator generators 30 and 32 are in non-oscillating conditionfor a short time while the brushes are both in contact with theconducting segment 1-8. When the timer moves so that the insulatedsegment 80 is under the brush 82, the operating coil 86 of thetube-oscillator generator 32 becomes deenergized, causing, first, aseparation of the contacts 12 and W4 of the transmission line 22 bylowering the hinged conductor section -70 thereof, and, then, a movementof the associated contact 5'4 to closed position, thereby placing thetubeoscillator generator in oscillating powerdelivering condition. Thelatter will deliver power until the conducting segment 18 reaches thebrush 82 whereupon its oscillations will be stopped and theconductor-section it of the transmission line 22 placed inshort-circu-iting position. Substantially immediately thereafter, thebrush 84 will leave the conducting segment 78 so that the operating coil96 of the tube-oscillator generator 38 is deener-gized, thereby firstcausing a separation of the contacts l2 and M of the transmission line26, and then moving its contact ttl to closed position so thathigh-trequency power is applied to the transmission line 20.

' From the foregoing, it is evident that with the timer rotating at aconstant speed, energy will be applied 'iirst to one transmission lineand then to the other transmission line for alternating repeatinginterva'ls, both lines being deenergized' for short times duringswitching. It is also evident that when one transmission line has powerdelivered to one of ends, the far end of the other transmission line isshort-circuited so that there is no voltage across its conductorsthereat. For manual control switches Hi and I62 can be used in the placeof the timer.

In accordance with the described form of my invention, the twotransmission lines have substantially the same electrical length and,under ideal conditions, each should be a quarter of a wave length at thefrequency at which the tubeosc-illator means "3E! and *32 oscillate, thefrequencies being preferably the same for both generators. Theelectrical length of the heating-electrodes with the material ittherebetween is also prefer-ably a quarter of a wave length at thesupplied frequency. This means that the total electrical length of theserially-connected transmission lines 29 and 22, with the dielectricheating means 2 therebetween, is three-fourths of a wave length.

with the generator end of a transmission line short-circ ited and thegenerator end of the other transmission line receiving power, standingwave Voltage patterns will be present along the power i 1,8 and L6,respectively, of the heating-electrodes.

The point F represents the point of the transmission line 20 at whichthe tube-oscillator generator 3t candeliver power thereto.

:Eaoh part of the curves A and B between the mints C and D, between thepoints D and E and between the points F and E, approximately a quanterof asine wave cycle, or a quarter of a wave length. Ihe portions of thetwo curves .A and B between the points D and E represent thestamina-wave voltage patterns along the heating-electrodes, and'areelectrical degrees out of phase. The successive voltages at each pointacross the material It can, accordingly, he represented as a tunctionof:the sine andcosine of the same angle. The heating or power input to thedielectric material depends on :the square of the voltage. Consequently,that part of the curves -A and B along the heating-electrodes must besquared for obtaining the heating along each point of the material. Ifthe curve between the points D and E is represented by the sine and thecurve B between the same points by the cosine, the squares are,-respectivelly, sine and cosine? From elementary trigonometry, the sineplus the cosine is equal to one, so that the average heating between thepoints 12) and E will, under the conditions assumed, be uniform alongall points if thestanding wave on the heating-electrodes isintermittently repeatedly changed from curve A to curve B and back atregular intervals.

' In actual practice, a heat-treatment can vary from several minutes toseveral hours as a rule. During each heat-treatment the wave-patternchanges cyclically in accordance with the speed of the timer. The speedof the timer '16 can obviously be controlledasdesired, to give as-man-yalterations of the standing-wave pattern on the heatingelectrodes asdesired during a heat-treatment.

The dielectric constant of the material being heat treated willgenerally vary noton-ly'for different batches of the material, but alsofora given batch during a heat treatment thereof, so that idealconditions are not generally obtained. However satisfactory uniformityis usually obtained with variations in the heating voltage on thedilierent points along the electrodes of as much as 2 to 5%, andsometimes more. Also many materials can tolerate somewhat highertemperatures above the minimum required for "heat-treatment. By makingthe minimum temperature occur at the minimum voltage-point, the materialcan be relatively quickly heat-treated without being adversely affectedby the higher temperatures present at the points of higher voltage.

'In order to heat materials of varying dielectric properties with asingle heating apparatus, it is desirable to adjust the supply frequencyso as to provide a suitable standing wave pattern on theheating-electrodes. The electrical length of the transmission lines canbe changed by adjusting their physical lengths and the tube-oscillatormeans moved in accordance therewith. To this end, each conductor of thetransmission lines comprises a pair of co-extending conductor-sectionshaving overlapping portions slidable on each other so that the totalphysical length of the transmission lines can be altered as indicatedschematically in Figs. 1 and 2. Such overlapping portions may heprovided with elongated slots permitting them'to be bolted together invarious eliiierent positions. When the length of a transmission line 2!]or 22 is changed, the cage containing the associated .tube andoscillator means 39 or .32 is correspondingly .moved. To permit this tobe conveniently done, the cages 34 and 38 are supported on wheels.

A somewhat different embodiment is shown in Fig. 4. In this embodiment,a single tube-oscillator means He is supported inside grounded metalcage 'H-z which is vertically adjustably supported from ceiling H4.Heating-electrodes 116, I18 and 420 are provided for heating dielec tricmaterial I therebetween. Three-conductor transmission lines I22 and I 24are provided respectively connected to opposite ends of theheating-electrodes. The natural resonance frequency of the transmissionlines and heating-electrodes can be adjusted by changing the amount ofoverlapping of the conductor sections thereof. The outer conductors aregrounded and the inner insulated conductors pass through opposite openwindows in the cage H2 for selective connection to the output side I26of an oscillator tube I28 of the tube-oscillator generator III]. Asuitable grid circuit I39 similar to that of the tube-oscillatorgenerators 36 and 32 is connected to the tube I28. The bias controlcircuit I32 for the grid includes a contact I34 controlled by a suitabletimer either to short-circuit, in effect, the negative bias or to permita high negative bias to be applied to the grid of the tube. Hingedsections I 36 and I33 on the insulated inner conductors of thetransmission lines are operable by the timer to short-circuit thetransmission line I22 while power is being delivered to the transmissionline I24, and for short-circuiting the transmission line I24 while poweris being delivered to the transmission line I22. The contact I34 and thehinged sections I36 and I38 are operated in suitable sequence so as tofirst stop the power delivery of the tube-oscillator generator Ho beforethe position of the hinged sections I32 and I34 are reversed.

In many cases, it is not especially essential to have quarter wavelengths of standing waves across the dielectric material, and in Fig. Ishow the heating which obtains with heating-electrodes of an electricallength, in operation, of 60" at the supplied frequency. Curve Mindicates the standing-wave voltage pattern when one transmission lineis energized and curve N the pattern when the other transmission line isenergized. Curve P is the average of the sum of the squares of thecurves M and N. This curve P is fairly fiat and indicates the markedimprovement in heating over that obtainable through operation with asingle standing-wave pattern, bearing in mind that the square of thevoltage is an indication of the heating.

The difficulty of generating high power at very high frequencies makesit desirable to keep the loaded length of the line, that is the lengthof the heating-electrodes between the transmission lines, down to aboutone quarter of the efiective wave length. This wave length, A, in feet,is given by the expression where f=frequency (cycles per second)K=dielectric constant of load.

With a block of plywood 4' Wide by 8' long by 6" thick, having adielectric constant of 4, between rectangular heating-electrodes ofabout the same dimensions, the frequency, from the foregoing equation,will be 15.4 megacycles if 8 feet of heating electrodes are required fora quarter wave (length, The length of the unloaded lines, that is thetransmission lines and 22, for example, to make up the other quarterwave lengths will each be approximately 16 feet, making a total lengthof about feet for the power transmitting and consuming means.

The flat plate-like conductors need not be very thick, but should be notless than twice the "depth of current penetration. At high frequenciesthe conductors actually can be very thin. The flat plate-like conductorscan store considerable reactive or circulating k. v. a. and are includedin the tank circuit for the tube-oscillator generators. In general, thecapacitance between the conductors makes the use of a tuning capacitorunnecessary or infeasible. Improved tuning is satisfactorily obtained bychanging the length of the transmission lines and changing the tuning ofthe tunable grid circuit to correspond.

While I have described my invention in forms which are now preferred, itis obvious that the principles and teachings of my invention havebroader application and can be readily utilized by those skilled in theart for other embodiments involving high-frequency heating.

I claim as my invention:

1. Dielectric heating apparatus for the highfrequency heating ofdielectric material, comprising, in combination, a plurality of spacedheating-electrodes for receiving dielectric material therebetween, aplurality of high-frequency transmission lines, a first of saidtransmission lines being connected to a first point on saidheating-electrodes and a, second of said transmission lines beingconnected to a second point on said heating-electrodes, said first andsecond points being spaced apart on said heatingelectrodes, generatormeans for delivering a high-frequency voltage to each transmission lineat a place removed from said heating electrodes, the frequency of thevoltage being sufficiently high to establish a standing-wave patternalong said transmission lines and said heating-electrodes, during aheating operation, said generator means comprising tube-oscillator meanshaving an oscillator tube and a frequency-determining circuit thereforcomprising said transmission lines, control means operable for causingpower from said tube-oscillator means to be delivered to each of saidtransmission lines, means operable for short-circuiting each of saidtransmission lines, and means for separately intermittently operatingsaid control means and said short-circuiting means in a predeterminedsequence during a heating operation.

2. High-frequency heating equipment comprising relatively insulatedspaced heating-electrodes providing a spacial field of energy forheating dielectric material within said field, said heating electrodeshaving a plurality of separate spaced places to each of whichhigh-frequency power can be established, and power applying means forcausing such power to be sequentially established at said places in apredetermined sequence, the frequency of said power and the distancebetween said places being such as to provide different curved voltagewave patterns on said heatingelectrodes, during a heating operation,with magnitudes of said standing wave-patterns such that the average oftheir squares is approximatelv uniform.

3. Dielectric heating apparatus comprising, in combination, a pluralityof spaced substantially rectangular heating-electrodes for receivingdielectric material therebetween, two high-frequency transmission lines,an end of each of said transmission lines being associated with aseparate one of a pair of opposite sides of said heating-electrodes, andmeans coupled to each of the other ends of said transmission lines, forselec-' tively applying high-frequency energy to each transmission linein a predetermined sequence,

9 the sequence for a first for said transmission lines being other thanthat for sequence for the other of said transmission lines, thefrequency of the energy supplied to said transmission lines being suchas to provide curved voltage Wave patterns on said heating-electrodes.

4. Dielectric heating apparatus comprising a plurality of spacedheating-electrodes for receiving dielectric material for heat-treatmenttherebetween, two generally similar high-frequency transmission lineseach comprising a plurality of spaced facing conductors, an oscillatortubemeans adapted to be electrically connected to either of saidtransmission lines, said heatingelectrode being electrically connectedto one end portion of each of said transmission lines, and means forcausing said tube-means selectively to deliver power to the other endportions of said transmission lines in a predetermined sequence.

5. Dielectric heating apparatus comprising a plurality of spacedheating-electrodes for receiving dielectric material for heat-treatmenttherebetween, two generally similar transmission lines each comprising aplurality of spaced facing plate-like conductors, an oscillatortubemeans adapted to be electrically connected to either of saidtransmission lines at a frequency determined primarily by saidtransmission line, said. heating-electrodes being electricallyassociated with one end of each of said transmission lines, and saidtube-means being electrically connectible to their other ends, meansoperable for short-circuiting each of said other ends, and timing meansfor intermittently alternately electrically connecting said tube-meanswith said transmission lines, and for operating said shortcircuitingmeans for the transmission line which is not electrically connected tosaid tube-means.

6. The invention of claim 2 characterized by said power-applying meanscomprising means for causing the wave patterns established on saidheating-electrodes to consist of substantially an odd number ofquarter-waves substantially fol lowing a sine and cosine form from apoint on said heating-electrodes, said power-applying means for applyingpower producing each of said sine and cosine forms alternately forsubstantially the same time periods.

7. A method of givin a dielectric material of some length a singledielectric heat-treatment between a pair of spaced heating-electrodes,the method comprising heating the material by applying high-frequencypower to the heatingelectrodes in a manner to establish a voltagewave-pattern thereon of varying magnitudes, and repeatedly cyclicallychanging the voltage wave-pattern by cyclically changing the networksystem connected to the heating electrodes 8. A method of giving adielectric material of some length a dielectric heat-treatment betweenspaced heating-electrodes. the method com rising alternately energizingthe heat ng-electrodes, for successive substantially equal time-periods,with high-frequency power providing a standingwave voltage pattern alongthe heating-electrodes, the wave having, in alternate periods, a sinefunction embracing a quarter-wave along a predetermined physical portionof the electrodes, and having, in the other alternate periods, a cosinefunction embracing a quarter-wave 10 length along substantially theidentical portion of the heating-electrodes.

9. A method of dielectrically heating a material we length between apair of spaced heatingodes, which method comprises successively ng theheating-electrodes with high-frequ 5 electrical power so as to establishfirst a highdrequency standing-wave voltage pattern along saidheating-electrodes, the pattern having an envelope which is a sinefunction of the frequency, and then, before the material has cooled,energizing the heating-electrodes with high-frequency electrical powerso as to establish standing-wave voltage pattern along saidheatlug-electrodes having an envelope which is a cosine function of thefrequency, that corresponds otherwise to the sine function.

10. Dielectric heating apparatus for the high- Irequency heating ofdielectric material, comprising, in combination, a plurality of spacedheating-electrodes for receiving dielectric material therebetween, aplurality of high-frequency transmission lines, a first of saidtransmission lines being connected to a first point on saidheating-electrodes and a second of said transmission lines beingconnected to a second point on said heating-electrodes, said first andsecond points being spaced apart on said heating-electrodes, generatormeans adapted to deliver a high-frequency voltage to each of saidtransmission lines at a place removed from said heatingelectrodes, thefrequency being sufiiciently high to establish a standing wave patternalong said transmission lines and said heating-electrodes, duringheating operations, and means comprising a timer for sequentiallycausing said generator means to apply energy to said transmission lines,with the sequence associated with said first transmission lines beingother than that associated with said second of said transmission lines.

ROBERT M. BAKER.

elem/f REFERENCES CITED The following references are of record in thefile of this patent:

UNITED STATES PATENTS Number Name Date 1,972,050 Davis Aug. 28, 19342,159,782 Conklin et all. May 23, 1939 2,293,533 Denneen et al. Aug. 18,1942 2304,958 Rouy Dec. 15, 1942 2,308,043 Bierwirth Jan. 12, 19432,308,204 Parry Jan. 12, 1943 2,370,423 Roberts Feb. 27, 1945 2,433,067Russell Dec. 23, 1947 FOREIGN PATENTS Number Country Date 118,453Australia May 11, 1944 OTHER REFERENCES Bierwirth et al.:Radio-frequency heating applied to wood gluing," Proceedings of theInstitute of Radio Engineers, October 1943, pages 529-537 (particularlypages 534 and 535). Copy in Scientific Library.

Batcher et al.: The Electronic Engineering Handbook (1944), ElectronicDevelopment Associates, East 46th Street, New York 17, New York, pages222 and 224. Copy in Division 60.

